I have seen them done with concrete vibrators, reciporicating saws, and power chisels/jack hammers. A plate is welded to the floor and the power tool has a blunt bit that strikes against it. Very effective. Zip tie the trigger in and use the power plug as the on/off. eventually the tool will wear through the floor but how often depends on throughput- figure once every other season. Note: set the tool up paralell to the walking surface striking a plate like I said earlier, you dont want a power tool shooting through into someones foot. with this set up a wear through just means it makes noise but no vibrations.
I hope that helps,
Allen H

I like the idea of a pneumatic powered setup because of the loud noise and vibration. I would think you could control this setup almost like you would if you were setting up a IR sensor break. So if you break the sensor it turns on the device and then off again maybe even throw in a BooBox or something for better control. One thing I would say is to weld a U shaped channel to the bottom of the panel so your vibrating device would hit that and never the panel your guests would walk on directly. If it ever wore through the channel then its just adding some more metal to the channel for a fix and would never be able to hit your guests. Just my thoughts on this idea but I think it would be a great startle scare. If you get it figured out I'd like to hear what you ended up with.